Category Archives: Screaming Circuits Blog

EMS provider Screaming Circuits offers tips and thoughts on design and manufacturing

Via Caps

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The best way of dealing with via-in-pad requirements is to have the vias filled and plated over at the board house. There can still be some issues with that technique, but if done right, is very reliable and the best way to go. But, what do you do for a Klondike bar? I mean what do you do if you can’t afford to fill and plate over your vias, or don’t have the time? Well, if your vias are in a BGA pad and you don’t want to have them filled and plated over, than you can just go back to the drawing board because your board most likely won’t work. There may be some secret, prototype-only things that your assembly house can do, but avoid it at all costs.

If your vias are under a QFN or SOT-23 or something bigger like that, you have more options. Don’t leave them open. Worms will use the open vias as homes. You can usually use soldermask to cap your vias in cases like this.

C is not an option. A and B can work. D can work also, but should be your last choice. With the tight masking like in A, most manufacturers recommend that the soldermask cap be about 100 to 125 micron wider than the diameter of the via. B is self evident – just make it bigger.

The pitfalls with D are that solder will still get sucked down into the via which can lead to excess voiding. Or, the via cap could pop open which results in effectively a C. With all soldermask via caps, top ot bottom, you need to check carefully to ensure that the soldermask gives a complete seal. And there is always a risk of the cap breaking. Ugh. Now do you see why everyone recommends filling and plating?

Duane Benson

I’m a capitalist and I’m okay…

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I’m a capitalist and I’m okay
I work all night and I sleep all day

Well, I don’t work all night so much anymore. I used to.  I am a capitalist though. I think money is good (what some people do with it, not so much). And, I also think that when people make money, they should do so in such a way that others benefit as well. That’s one of the reasons I like Ti’s Beagleboard so much. Speaking of Ti, they have another microcontroller product that I’m excited about as well.

The MSP430 LaunchPad is a little development board designed for education in general, as well as familiarization with the MSP430 line of microcontrollers for experienced developers. (I hope they don’t mind that I’m using their picture. I don’t have one so I couldn’t take my own picture of it.)

A lot of companies have development boards for their chips. That’s nothing new. But what is really cool is that they have set a retail price for this of $4.30. Yes, the price of a 16 ounce latte.

Now I know that a latte is important. Some people have speculated that civilization would collapse without caffeine. But, here’s what you get in the place of that latte: (from the Ti website, again)

“For $4.30, the LaunchPad includes a development board, 2 programmable MSP430 microcontrollers, mini-USB cable, PCB connectors for expandability, external crystal for increased clock accuracy, and free & downloadable software integrated development environments (IDEs)”

Cool. I can explode one and still have another to finish the project with. I’m going to get me one of these and spend some time with it. I don’t have any personal experience with the MSPP430 line, so it will be filling it’s primary mission.

On the subject, I ran across an interesting website dedicated to the MSP430: www.43oh.com. If you’re already a 430er or are just intrigued by the chip, go check it out.

Duane Benson
Buttered scones, anyone?

Monsters Under the Bed

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TO-263 land with mistakes Monsters, metaphorically speaking, that is. Take a look at this little land pattern for a TO-263 part. Can you tell me the two main things wrong with this land pattern?

I’ll give you a hint. One of the problems is an absolute no-no. The other one could be justified with a low-current application. But then, wouldn’t you use a smaller package?
Duane Benson
Green Grow the Traces Ho

http://blog.screamingcircuits.com/

Open Source Mea Culpa or Backpedaling? You Decide

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Last week I wrote about “Ten electronics things to be thankful for in 2010.” If you’re reading this now, you probably don’t need the link because you probably read that article then, too. But that’s not really relevant. What is relevant is that in #6, about open source hardware, I wrote, in part, that in some ways open source seems a bit exploitative of the designers.

In the comments, “pt” asked if I could give an example of how open source is exploitative.

I got to thinking about my choice of words and came to the conclusion that “exploitative” doesn’t quite cover what I was trying to say. Although, in some cases, I think it does. It’s possible that there are some aspects of the open source movement that I just don’t get. Or it’s possible that I have the capacity to pick a black cloud out of anything. If that’s the case, I like to think that I can also pick a silver lining out of anything as well. That combination becomes a problem with recursion.

I’m a capitalist so I believe that (a) it’s important to have profit as an ultimate goal of any commercial endeavor. I still have a bit of idealism left so I also believe that (b) when making that profit, we should be like the Boy Scouts and leave the planet a little better off than we found it. I get sick to my stomach when I read about executives making massive millions of dollars when their employees struggle to adequately feed and clothe their kids. (Is this post turning into one of those “I believe …” manifestos?) Following up that last point, I believe that (c) if someone does good work, they should get something in return for it. It’s a trade. You give me something valuable and I’ll give you something valuable in return. Not always money, but something of value.

That’s where the mushiness comes in for me. Here’s the good side. Ti is a big company that, with the Beagleboard, is giving something of great value to the electronics design community. As far as I know, the people at TI working on the project are paid. My guess is that the ultimate motivation of TI is it to help sell chips, but the project has given a whole lot of people access to a level of performance whom would not have had access it otherwise. That endeavor meets my abc conditions. Companies like Adafruit, Sparkfun and DIYdrones have built successful small (and growing) businesses with the help of open source hardware and software. People are making a living (I assume) from those organizations. Both companies give a lot back to the community and both companies make it very clear that they benefit from and really appreciate the efforts of open source designers. They give the folks recognition and support. They and companies like them meet my abc.

The other side of open source, and where I smell the exploitation, is when big companies use open source, make large profits and don’t return anything. I mean, sure, the license allows them to and I suppose that by reducing their costs, they can be more competitive and stay in business, keeping their employees employed. But when a software company buys the remains of another company or two that allegedly “own” some open source code and then tries to make a business of suing people that use that open source software; I consider that to pretty exploitative of all of the people that voluntarily gave their time to the project.

When a large muti-national company that sells server farms uses an open source OS and doesn’t return anything to the designers, I find that also to be exploitative. I don’t know what the answer is. I mean it’s cool that Linux, for example, is used in so many places. The fact that big corporations put so much weight on it certainly validates the legitimacy of it. But I can’t help but envision open source developers out there, that could really use a bit more money in the bank, looking at those big corporations that are profiting off of their backs, feeling a little used.

So, am I missing something? Do I not get it?

By the way, this piece has a lot of personal opinion in it, but I do believe that my company works hard to meets my abc so I don’t have a problem posting this on my work blog. The two times in my career that I did work for companies not meeting my abc, both ended badly for me. Fortunately, I believe in this one.

So, help me out here. If I’m not getting a part of this, feel free to chime in.

Duane “Does idealism hold up in the face of reality?” Benson

http://blog.screamingcircuits.com/

Narcissistic Parts

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Maybe not completely narcissistic, but at least self-centered.  Or, self-centering. Okay, are you lost now? Am I making any sense at  all? Well, I’m going to say that it doesn’t matter, because the  world-revolves around me.

But what I am talking about is parts that will more or less center themselves during the reflow process. Some parts like BGAs and QFNs tend  to follow the surface tension of the melted solder and tweak themselves  into a more centered position on the land. That’s a good thing.

It’s not always a good thing though. Sometimes that same surface  tension action can work against you. Take this TO-263 (pictured). When it was placed on the land, before reflow, the leads were centered  right in their pads like they should be. The big land for the thermal  pad is set up a little too high though and once melted, the surface tension from the big thermal pad sucked the part up, nearly dragging it  off of the lands for the leads. Bummer days. Here’s another example.

You probably shouldn’t leave the part like this, so here’s a few suggestions.

You could make the thermal pad smaller so that when the metal tab of  the part is centered, the leads will be, too. Cooling needs might dictate  that you don’t reduce the size of the pad, though. If that’s the case,  you could make the bad bigger by extending it down toward the leads, again so the leads will be centered when the body of the part is. You could also mask off the top part of the pad, or put a thin strip of mask  as a solder dam. What you’re doing is making sure that if and when surface tension moves the part, the leads will end up where they are supposed to.

<em>Duane Benson
It is all about me, you know</em>

http://blog.screamingcircuits.com

0.4mm Pitch BGA Land Patterns

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We’ve been getting more questions about laying out the 0.4mm pitch Ti OMAP BGA, as is used in the Beagleboard. As I’ve written before, some of the rules change at these tiny geometries. The Beagleboard folks discovered that non soldermask defined pads (NSMD) can lead to bridging and poor yields and therefor they recommend soldermask defined (SMD) pads. Check out page 10 in their design guide. If in doubt, or if you’re concerned that your set up might be different enough to warrant NSMD pads, I’d suggest you give a Ti Applications engineer a call.

And speaking of the Beagleboard, they just recently reduced the price on their pre-built Beagleboards. Like $125 for the original and $149 for the new xM version. Very nice.

If you’ve got a 0.4mm BGA part from a different manufacturer, check with that particular part manufacturer for the final say. Some 0.4mm pitch parts have a staggered arrangement and in that case, there is enough room center to center to successfully use NSMD pads.

Duane Benson
Joe Cool here.

http://blog.screamingcircuits.com/

10 Electronics Things to be Thankful for in 2010

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“Do they have 4th of July in Canada?”

The Thanksgiving holiday is upon those of us here in the United States. It’s been a bummer of a couple of years for a lot of the electronics world, but there’s still plenty to be thankful for — and I think it’s getting better. Well, “better” is a relative term, I guess. We at Screaming Circuits have gone from feeling the effects of the recession to being overwhelmed with work as people get back to designing stuff.

Here’s my recommendations on what to be thankful of this holiday season. Feel free to come up with your own list. I won’t look down on you if you don’t use my exact list.

#10: SIlicone. Because, while germanium is a semiconductor, silicon works much better. Germanium can’t stand the heat and had to get out of the fire.

#9: Flip chips. They’re so tiny and cute. And they have better thermal transfer properties than wire-bonded chips, not to mention improvements in inductance. And you can jam a whole lot more into the same space with little flip chips than you can with SOIC chips. Plus, if you run out of pepper, you can season your mashed potatoes with a bunch of spare flip chips. Just make sure they’re lead-free.

#8: HASL. Yes. It’s still around. And while it’s not the best solution for the aforementioned flip chips, it is one of the most robust, easiest to store, handle and use when dealing with larger geometries. It’s the way to go when hand soldering.

#7: ENIG and immersion silver. HASL may be my preference for hand soldering, but when using big BGAs or lots of small components, the bumpy surface of HASL can cause problems. That’s when a nice planar surface such as ENIG or immersion silver makes life a lot easier.

#6: Open source hardware. Open source has been helping out the software industry fore quite a while. It’s about time hardware folks benefited from the concept. In some ways it seems a bit exploitative of the designers, but as long as they are doing it voluntarily, I guess it’s okay. Open source hardware gave us the Arduino, which seems to have made microcontrollers a lot more accessible. It gave us DIY Drones, which seems to be proving that autonomy isn’t just for big-iron.

#5: mBed. This nice little ARM development board has taken a new approach to dramatically reducing the barriers to entry. With a complete online IDE and extremely easy startup and use, it will help a lot of people learn about advanced microcontrollers and will help a lot of people move from 8-bit up to the 32-bit ARM world. I don’t think you could make it any easier than this.

#4: FTDI. They made USB easy to implement on just about any design. Cool.

#3: The Beagleboard-xM. Speaking of open source hardware, the Beagleboard came about a few years ago as the first (as far as I could tell) seriously powerful open source hardware platform. It brought open source out of the hobby garage and into corporate America. The New xM has made the design even more powerful and indicates Ti’s commitment to the project.

#2: Quickturn PCB fab and assembly houses. Like Screaming Circuits for assembly and our buddies at Sunstone for the PCB fab, so you can get your prototypes built up a lot faster. Yes, I know this one is self-serving. But, you know, these guys pay my salary and I really believe in what we do here.

#1: Drum roll please…

#1: Caffeine. It helps us keep designing into the wee hours of the night. Then it helps us get back to designing early in the morning when we should be sleeping because we stayed up to late the night before. Caffeine is the fuel that powers our economic engine, so that’s my #1 thing to be thankful for this Thanksgiving. There is a part of me, however, that suspects that due to caffeine, we might just be doing this all wrong. Maybe we should, instead, try actually sleeping the proper number of hours per night. Just a thought.

Duane Benson
Wikipedia says caffeine is a natural pesticide. Hmmm…
Well, at least it’s natural.

http://blog.screamingcircuits.com/

Land Patterns – Equal and Not Equal

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I was recently asked a question about QFN package varieties. The questioner wanted to know if different package variants of 16 contact QFN packages, such as HUQFN, DHVQFN, SQFN and such, all shared the same footprint.

If they did, the CAD work would be much easier. There would be one land pattern to worry about and that would be that. Unfortunately, that is not that and in this case, that, in fact, that may never be that.

Many different varieties of QFN packages could use the same land pattern, but they don’t always do. Some will have the same pitch, but more distance between the outside contacts and the corner, thus a greater overall dimension. That can happen even with the same labeled variety of QFN package. Some will have different dimensions, differnt pitch, different pad sizes or different thermal pad sizes. Sorry. No easy answer here.

I popped on over to the NXP website, one of our Circuit Design ECOsystem partners, for some examples. NXP lists two 60 contact HUQFN part packages. One is 5 x 5 mm. The other is 6mm x 4mm. Same with the HVQFN. There is a 0.65 mm pitch, 4 x 4mm package and two 0.5 mm pitch, 3 x 3mm parts with a different overall package outline.

In general, generalzations aren’t going to work here. You’re going to have to go dig out that datasheet and quite possibly create a new land pattern.

Duane Benson
One pattern to rule them all and in the solder bind them

http://blog.screamingcircuits.com/

Need a Reference for the Reference

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Not long ago, I wrote a short post about non-standard use of reference designators. After doing that, I’ve been looking at some of my own microcontroller and motor driver boards with an eye for how close to standards I am.

All of the R’s, C’s, D’s and U’s are okay, but there are some differences. For example, the Eagle library I’ve been using calls crystals “X” instead of the more standard “Y.” I have seen crystals designated as “X”, “Y” and “Q.” LEDs seem to go by “LED” instead of “D” as indicated in the Wikipedia list. Headers go by “J”, “JP”, or “H.” Wikipedia says “J” is for a female jack connector, “JP” is for jumper, and it doesn’t list a “H.” My board has break away two-row male headers and keyed single-row male headers. Wikipedia does note that its list is a set of commonly used designators. Not necessarily standard.

We probably do have the specific standards document laying around here someplace, and if I were doing real work on a professional basis, I’d hunt it down and make sure I followed the actual standards. But I’m not doing real work with my controllers and drivers, so I just do the best I can. I wonder how often that happens everywhere. The standards books are “somewhere” but no one really knows where.

Duane Benson
Somwhere over the reflow…

http://blog.screamingcircuits.com/

Faster, Chug-a-Chug … Faster, Chug-a-Chug …

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Ever long for the days when you could lazily send out your files to get boards fabbed and a prototype assembled and then have a leisurely couple of weeks waiting for it all to completed and returned? Well, we’re not going to help you get back to that. In fact, we try to do the opposite. Let’s make everything speedier and speedier. I hope that’s okay.

Screaming Circuits’ PCB fab partner, Sunstone Circuits just added in a bit more to that end. Back in the old days, if you needed PCBs fabbed in 24 hours, you had to stick with two-layer boards. Not any more. They recently started offering four-layer PCBs fabbed in 24 hours in their PCBexpress quick-turn service. No rest for the weary. That’s especially cool if you’re having signal integrity problems and need to add in a ground and/or power plane layer.

Duane Benson
If by approaching the speed of light, time speeds up for you,
does time slow down for you as you approach “stopped”?

http://blog.screamingcircuits.com